Pureline Selection – Procedure, Applications, Advantages, Disadvantages

What is Pureline Selection?

• Pureline selection is a breeding method primarily used in self-pollinated crops. A pureline is the progeny derived from a single homozygous plant that has undergone repeated self-pollination. This process ensures that all individuals within the pureline have identical genotypes, with any observable variation being attributed solely to environmental factors.
• In pureline selection, a large number of individual plants are selected from a population of a self-pollinated crop. Each selected plant is harvested separately, and its progeny are evaluated. The best-performing progeny is then chosen and released as a pureline variety. Therefore, this process is also known as individual plant selection, as it focuses on the selection and propagation of single, genetically uniform plants.
• Pureline varieties are distinct from those developed through mass selection, which consists of a mixture of different purelines. In contrast, pureline selection yields a uniform variety, where each plant shares the same genetic makeup. This method is favored in self-pollinated species due to the genetic consistency it provides, resulting in pureline varieties that are more commonly developed in such crops than mixtures of purelines.

Definition of Pureline Selection

Pureline selection is a plant breeding method where individual plants are selected from a self-pollinated crop, and their progeny are evaluated. The best-performing progeny, derived from a single homozygous plant, is released as a pureline variety, ensuring genetic uniformity.

History of Pureline Selection

• Early Development (Pre-19th Century) Pureline selection has its roots in agricultural practices that predate the mid-19th century. Early methods of individual plant selection were employed to improve crop varieties, though systematic approaches were not yet formalized.
• Mid-19th Century Advances (1840-1860) During the period between 1840 and 1860, several pioneering agriculturists, including Le Couteur, Shireff, Hallett, and the Vilmorins, advanced the practice of individual plant selection. This approach was applied to various crops such as wheat (Triticum aestivum), barley (Hordeum vulgare), and sugar beets (Beta vulgaris). These early efforts laid the groundwork for modern pureline selection techniques.
• The Vilmorin Principle One of the significant contributions to pureline selection came from the Vilmorin family, particularly through the work of Lout de Vilmorin. He proposed what is now known as the Vilmorin Principle or Vilmorin Isolation Principle, which forms the basis of progeny testing. This principle emphasized the need to isolate and evaluate individual plants to ensure genetic uniformity.
• Establishment of the Svalof Experiment Station (1806) The Svalof Experiment Station, established in 1806 by the Swedish Seed Association, played a crucial role in refining pureline selection techniques. Researchers at this station developed and formalized the practice of pureline selection, contributing significantly to its current methodologies.
• Genetic Explanation (1903) The genetic foundation of purelines was clarified by Wilhelm Johannsen in 1903. Johannsen’s work provided a scientific basis for understanding how purelines maintain genetic consistency and how they differ from other selection methods.
• Modern Application Pureline selection has become the predominant method for improving self-pollinated crops. Today, most of the commercially available varieties of self-pollinated crops are developed through this method, reflecting its efficacy and widespread adoption in crop improvement.

Characteristics of Pureline Selection

• Genetic Uniformity
  All plants within a pureline share the same genotype, as they originate from a single homozygous, self-fertilized parent. This ensures that the genetic composition is consistent across the entire population.
• Environmental Variation
  Any variation observed within a pureline is due to environmental factors rather than genetic differences. Since the genotype remains identical in all plants, changes in traits are strictly non-heritable and influenced by external conditions.
• Potential for Genetic Variation Over Time
  Although purelines are initially genetically stable, they may become variable over time due to factors like mechanical mixtures, natural hybridization, or spontaneous mutations. While the first two can be managed through careful cultivation practices, mutations introduce unavoidable genetic variability, requiring periodic efforts such as mass selection to maintain purity.

Procedure of Pureline Selection

The process of pureline selection involves a series of systematic steps aimed at isolating and developing pureline varieties from a mixed population. The procedure typically spans several years and can be adjusted based on specific needs. Below is a detailed description of the general procedure:

1. Step One: Selection of Individual Plants
   • First Year: A large number of plants, ranging from 200 to 3,000, are selected from a local or mixed population. These plants are harvested separately to maintain genetic distinctness. The selection should be conducted with adequate spacing to facilitate individual plant evaluation.
   • Criteria for Selection: Plants are chosen based on observable traits such as flowering time, maturity, disease resistance, and physical characteristics like plant height and presence of awns. This initial selection aims to capture superior genotypes from the population.
   • Considerations: The number of plants selected should be maximized within the constraints of time, land, and resources. A larger sample size generally increases the likelihood of identifying desirable traits. Careful selection ensures that the progenies from these plants will be evaluated based on the genetic variation among them rather than within each plant.
2. Step Two: Evaluation of Progenies
   • Second Year: Progenies from the selected individual plants are grown separately with sufficient spacing. This stage focuses on reducing the number of progenies that will advance to the next step.
   • Evaluation: Progenies are visually assessed and evaluated for traits such as plant type, height, grain characteristics, and disease resistance. Poor or defective progenies, as well as those with excessive segregation, are discarded.
   • Selection Criteria: Selection is based on easily observable traits. Disease epiphytotics may be induced to test for disease resistance. The goal is to further refine the progenies to ensure only the most promising ones are carried forward.
3. Step Three: Yield Trials
   • Third Year: A replicated yield trial is conducted to critically evaluate the selected progenies. This includes planting the best available variety as a check and comparing it with the new progenies.
   • Evaluation: Progenies are assessed for yield, disease resistance, and other observable traits. A preliminary yield trial may be conducted if sufficient seed is available.
   • Objective: The main aim is to further reduce the number of progenies by selecting those with the best performance. Each progeny is considered a pureline, making further selection within progenies unnecessary.
4. Step Four: Advanced Yield Trials
   • Fourth Year: Replicated yield trials are continued, using the best available variety as a reference. Detailed observations are made on disease resistance, flowering and maturity times, and other characteristics.
   • Quality Tests: Additional tests may be conducted to assess quality characteristics of the progenies. The best-performing strains are identified and included in coordinated yield trials.
5. Steps Five to Eight: Multi-Location Trials
   • Fifth to Eighth Years: Promising strains are evaluated across multiple locations along with strains from other breeders. This multi-location testing helps to confirm the consistency and performance of the strains in different environments.
   • Objective: The strains are compared to the best released varieties, and performance data is collected to support further selection.
6. Step Nine: Release of New Variety
   • Ninth Year: The most promising progeny or strain is officially released as a new variety. This decision is based on comprehensive evaluation data from all previous stages.

Use of Pureline Selection

• As a Variety
  Purelines can serve as superior varieties for self-pollinated crops. The majority of modern varieties in these crops are derived from purelines, which are valued for their genetic uniformity and stable traits.
• As Parents in Hybridization Programs
  Purelines are essential in hybridization programs, even if they are not ideal for commercial release. They provide consistent genetic backgrounds, which are crucial for developing new varieties through controlled breeding techniques.
• In Studies on Mutation
  Purelines are used in research on spontaneous or induced mutations affecting quantitative traits. Since purelines have a uniform genotype, any observed genetic variation can be attributed to mutations, provided that mechanical mixtures and natural hybridization are controlled.
• In Biological Research
  Purelines, or highly inbred lines, are important in various biological studies, including medicine, immunology, physiology, biochemistry, and nutrition. These lines ensure that experimental outcomes are not influenced by genetic variability, allowing researchers to accurately detect and interpret the effects of treatments.

Applications of Pureline Selection

• Improvement of Local Varieties Pureline selection is particularly effective for enhancing local or traditional varieties with significant genetic variability. This method has been used to develop numerous varieties. For instance, pureline selections such as NP 4 and NP 52 in wheat (Triticum aestivum), NP 1 and NP 12 in linseed (Linum usitatissimum), and T1 cowpea (Vigna anguiculata) have been derived from local varieties. Additionally, Pusa Sawani bhindi (okra) is a pureline selection from an old local variety in Bihar, demonstrating field resistance to yellow mosaic virus.
• Selection from Introduced Varieties Pureline selection is also applied to materials introduced from other regions or countries to create suitable new varieties. Examples include Shining Mung I, which was selected from an introduced variety Kulu Type 1, and PS 16, derived from an introduction from Kalyan Sona. Another notable example is the leaf rust-resistant variety Kalyan Sona, selected from an introduction by CIMMYT, Mexico. This approach helps in adapting introduced materials to local conditions.
• Improvement of Old Pureline Varieties Over time, pureline varieties can become genetically variable due to factors such as mechanical mixture or mutation. Pureline selection can be used to isolate new, stable varieties from these genetically variable populations. Examples include Chafa (derived from No. 816 gram), Jalgaon Baisathi (from T44), and CO 2 (from P 160) mung. Off-type plants, which deviate from the original pureline characteristics, can also be selected to create new pureline varieties, such as the dwarf variant Shyama, selected from the tall pureline rice variety Kalimoonch 64.
• Selection for New Characteristics Pureline selection can address the need for new characteristics not previously emphasized. For example, the jowar (Sorghum bicolor) variety resistant to root rot caused by Periconia circinata was developed after the disease became widespread in Kansas in 1926. Resistant plants were selected from an infested field to create a new root rot-resistant variety.
• Selection in Segregating Generations Purelines can also be selected from segregating generations resulting from crosses. Various methods, such as pedigree, bulk, or backcross methods, are used to isolate purelines in these generations. These methods, which are essential for developing stable pureline varieties, will be discussed in greater detail in subsequent chapters.

Advantages of Pureline Selection

• Maximum Improvement Over Original Varieties
  • Detailed Improvement: Pureline selection enhances the genetic quality of a crop by isolating and propagating the best possible genotype from a population. As a result, the variety developed through this method represents the peak of genetic improvement available from the original population.
• Uniformity of Varieties
  • Consistent Genotype: Pureline varieties exhibit a high degree of uniformity because all plants in the variety possess identical genotypes. This uniformity ensures that the crop performs consistently across different growing conditions and simplifies management practices.
  • Farmer and Consumer Preference: The consistent characteristics of pureline varieties make them more appealing to farmers and consumers. This preference is due to the predictability in traits such as size, color, and yield, which are critical for commercial production and market acceptance.
• Ease of Identification in Seed Certification
  • Simplified Certification: The extreme uniformity of pureline varieties facilitates their identification in seed certification programs. Because all plants within a pureline are genetically identical, distinguishing the variety from others is straightforward, enhancing the efficiency of certification processes.

Disadvantages of Pureline Selection

• Limited Adaptation and Stability
  • Narrow Adaptation: Pureline varieties often lack the broad adaptability and environmental stability of local or traditional varieties. This limitation is due to the fact that pureline selection focuses on optimizing specific traits from a particular genetic background, which may not be as versatile in varying environmental conditions.
• Resource-Intensive Procedure
  • High Costs and Resource Needs: The process of pureline selection demands considerable time, space, and financial resources. Compared to mass selection, which is generally less resource-intensive, pureline selection involves extensive yield trials and detailed evaluations, increasing the overall costs and complexity of the breeding program.
• Genetic Improvement Constraints
  • Genetic Variation Limits: The extent of improvement achievable through pureline selection is constrained by the genetic variation present in the original population. Once the genetic limits of the starting material are reached, further enhancement of the variety is not possible, which can restrict the potential gains from this method.
• Increased Time Commitment
  • Time Allocation Issues: Pureline selection requires a significant time investment from breeders, which can detract from their ability to engage in other breeding programs. This increased time commitment may limit the breeder’s capacity to simultaneously manage multiple projects, affecting overall productivity.

Achievements of Pureline Selection

Pureline Selection has significantly contributed to the development of improved crop varieties, particularly in the early stages of crop improvement work in India. The method capitalized on the genetic variability present in local or desi varieties, leading to substantial advancements in various crop species. The following points outline the notable achievements of pureline selection:

1. Widespread Use and Development:
   • Pureline selection was extensively employed in the early crop improvement programs in India, leveraging the genetic variability of desi varieties.
   • It led to the creation of a large number of improved varieties across several crop categories.
2. Success in Self-Pollinated Crops:
   • Cereals: Significant improvements were made in wheat (Triticum aestivum), barley (Hordeum vulgare), and rice (Oryza sativa).
   • Pulses: Varieties of gram (Cicer arietinum), mung (Vigna radiata), and urid (Vigna mungo) were developed.
   • Oilseeds: Improved varieties of groundnut (Arachis hypogaea) and linseed (Linum usitatissimum) were created.
   • Brassica Species: Self-compatible species like rai (Brassica juncea) and toria (Brassica campestris var. toria) were improved.
3. Successful Examples in Crop Varieties:
   • Wheat: Varieties such as NP 4, NP 6, NP 12, Coimbatore 2, Gadag 1, and MCU 1 were developed.
   • Rice: Varieties including Mtu 1, Mtu 2, Mtu 7, Ar 1, S155, BRL, BR3, Patni 6, Waner, T1, T3, T22, and T29 were notable examples.
   • Barley: Varieties like C 251, C 50, and K 12 were introduced.
   • Tobacco: Varieties such as NP 28, NP 63, NP 70 (Nicotiana tabacum), and Harrison Special 9, Chatham (Nicotiana rustica) were developed.
4. Notable Achievements in Specific Crops:
   • Mung: Varieties T1 (from Muzaffarpur, Bihar) and B1 (from Chaitali Mung) were successfully developed.
   • Urid: Varieties like T9, T27, Naveen, Kulu 4, and T122 originated from collections across various regions.
   • Cotton: The introduction of Gossypium hirsutum from the U.S.A. led to the development of Dharwar-American, which was later improved to Gadag 1. Gadag 1 exhibited enhanced yield and agronomic characteristics but had susceptibility to leaf blight. Buri 107, another pureline variety, showed improvements in staple length, spinning quality, and disease resistance.
   • Tobacco: Pureline selection resulted in HS 9 and Harrison Special varieties, both showing increased yield and uniformity compared to the original Harrison Special. Chatham, selected from culture 40D, exhibited superior cured leaf color and suitability for late planting.
5. Impact on Agriculture:
   • The varieties developed through pureline selection significantly influenced Indian agriculture, providing higher yields, better quality, and improved uniformity. These achievements underscore the method’s effectiveness in enhancing crop performance and meeting agricultural demands.

Comparison Between Pureline Selection and Mass Selection